Mike . . . I'd say your have two basic options: Pick a color/value very close to the color/value of the material on which it is painted so it is unobtrusive, or pick a color/value closer to the color value of her hat, sweater or her flesh tone, making your signature a design element within the painting. Of course, there are other options, but even so, my thought is you either want your signature to be part of the composition/design, or you want it not to stand out. Just my thoughts.

Well . . . one thing I might try is to find some cardboard, or colored paper that is close in color and value to your canvas. Then, I'd do several signatures in various color/values. Then, like a jigsaw puzzle, lay them on the painting and move them around until, like an English Pointer on a quail, something locks in, and you cry out, "Zeus!!! I've got it!!!"

I'm with Richard in making it a design element, something I do frequently. As the work becomes increasingly sketchy in the lower third, I personally would make the signature "vertical-ish", moving upward to meet the silouhette line of the breast (as if the contour of the breast were continuing downward. It would be dark enough to be distinct, but light enough to be unobtrusive.

Mike, I was thinking of something more like this, i.e. as if the signature was a continuation of the contour line. (I've used a "generic" unrecognizable signature to illustrate what I mean. It could go darker or lighter.)